Electric motors convert electricity into rotating motion using magnetism. In a conventional electrical motor, there are two parts: a fixed part called the stator and a rotating part called the rotor.
Electrical Motors can be divided into DC motors and AC motors. Within the AC Motors category, there are two broad classes: Induction Motors and permanent magnet synchronous motors (PMSM). Within the DC motors category there are two broad classes: Brushed Motors and Brushless Motors.
In each of the motor classes, the rotor and stator have magnets with opposing magnetic fields. The magnetic field can be created with permanent magnets or with electro-magnets. Typically at least one set of the magnets are electro-magnets so that the electro-magnets can be sequentially energized, creating a rotating magnetic field. This rotating magnetic field results in the magnetic poles on the rotor and stator pulling and pushing to the nearest magnetic pole. The process of continual sequentially energizing the electro-magnets to create a rotating magnetic field is call “commutating”. In a brushed DC motor, the rotation is accomplished via a mechanical commutator, in which case the term “self-commutating” is used. For brushless DC motors, where the commutation is performed by a motor controller, the term “external commutation” is used.
When a rotor is mechanically rotated, the magnetic field interaction between the rotor and stator generates a voltage. A motor used in this fashion is called a generator. The faster the rotation of the rotor, the higher the generated voltage. When a rotor is turned electrically, the magnetic field interaction between the rotor and stator creates a voltage just as if the motor was a generator. The voltage created corresponds to a force that opposes the voltage used to drive the electromagnets in the motor. This force has several names, two common names are “counter-electromotive force” and “back-electromotive force” or “BEMF.” The term BEMF is used herein to refer to this induced voltage. The BEMF voltage has a magnitude proportional to the rotational speed of the rotor.
In a brushed DC motor, the rotor has electro-magnets, and commutation is accomplished mechanically with a segmented contact on the rotor shaft where the various electro-magnets of the rotor are connected. A brush, typically a carbon block, conducts current to the segments on the commutator that energizes the electro-magnets on the rotor. As the rotor turns, the brushes move from one to another set of contacts, energizing another set of electro-magnets. Sequential energizing of the electro-magnets causes the rotor to turn. In the case of a brushed DC motor, simply supplying DC power to the motor will cause the rotor to turn.
In a brushless DC motor, the rotor has permanent magnets and the stator has electro-magnets. The stator windings are externally commutated by a motor controller that uses the sensed speed and position of the rotor to time the energizing of the electro-magnet coils so as to cause the rotor to turn. In some externally commutated motors Hall Effect sensors are used by the motor controller to sense the position and speed of the rotor.
As the rotor turns, a BEMF is generated in the stator that is proportional to the speed of the rotor. Some motor controllers measure the BEMF voltage to sense the position and speed of the rotor. This approach eliminates the need, cost and size of the Hall Effect sensors. Motor controllers using this technique are sometimes called “sensorless” controllers. Product applications that can take advantage of the smaller, less costly sensorless controllers benefit from the sensorless technique. Further improvement and simplification of the motor/controller devices is an ongoing effort.